In local parlance, the great apes of the Bili Forest fall into two distinct groups. There are the "tree beaters", which disperse high into the trees to stay safe, and easily succumb to the poison arrows used by local hunters. Then there are the "lion killers", which seldom climb trees, are bigger and darker, and are unaffected by the poison arrows.

When Karl Ammann, a Swiss photographer and anti-bushmeat campaigner, first visited the region in 1996, he was looking for gorillas, but instead discovered a skull that had dimensions like that of a chimpanzee, but with a prominent crest like that of a gorilla. Ammann purchased a photograph, taken by a motion-detecting camera, from poachers that captured an image of what looked like immense chimpanzees. Ammann also measured a fecal dropping three times as big as chimp dung and footprints as large as or larger than a gorilla's.

In 2000, Ammann returned to the area described by the bushmeat hunter with a group of ape researchers. Although they did not find a live Bili ape, they did find several well-worn ground nests, characteristic of gorillas rather than chimpanzees, in swampy river beds.

Since a five-year-long civil war ended in 2003, it has been easier for scientists to conduct field research in the Congo. The first scientist to see the Bili apes, and also recruited by Ammann, was Shelly Williams, PhD, a specialist in primate behavior. Williams reported on her close and chilling encounter with Bili apes, "We could hear them in the trees, about 10 m away, and four suddenly came rushing through the brush towards me. If this had been a mock charge they would have been screaming to intimidate us. These guys were quiet, and they were huge. They were coming in for the kill – but as soon as they saw my face they stopped and disappeared."[7]

"The unique characteristics they exhibit just don't fit into the other groups of apes", says Williams. The apes, she argues, could be a new species unknown to science, a new subspecies of chimpanzee, or a hybrid of the gorilla and the chimp. "At the very least, we have a unique, isolated chimp culture that's unlike any that's been studied", she says.[8]

Scientists believe they are dealing with a very inbred population, in which even a large number of animals could share identical or near identical haplotypes. Bili ape reports have also been investigated by Esteban Sarmiento, who has said "I would think there is a strong possibility that south of Bili on the other side of the Uele River there may be gorillas, and this would seem an important area to turn our attention to." Scientists working within these forests south of the Uele, however, have found no such evidence, nor heard any such reports from local communities. It remains an important region, however, based on the discovered presence of other flagship species, like chimpanzees and elephants.[9]

In June 2006, British Science Weekly reported that Cleve Hicks and colleagues from the University of Amsterdam had completed a year-long hunt for these apes during which they were able to observe the creatures a total of 20 full hours. Hicks reported,"I see nothing gorilla about them. The females definitely have a chimp's sex swellings, they pant-hoot and tree-drum, and so on."[10] DNA samples recovered from feces also reaffirmed the classification of these apes in the chimp subspecies Pan troglodytes schweinfurthii.

Hicks encountered a large community of the apes to the northwest of Bili that displayed interest in him and his colleagues reminiscent of previous reports (this was misreported in the New Scientist as being 18 km from Bili, but it was actually considerably farther from the village. This was the first group of Bili apes to be encountered where the adult males did not flee immediately upon seeing the humans). The apes, including adult males, would surround their human visitors and show curiosity towards them, but would not attack or become threatening.[11][12]

Hicks has emphasized that there is little evidence suggesting that they are any more aggressive than other chimpanzees (predatory behavior being the norm for the species). However, he has been misquoted in the press about this.[13]

Further study was undertaken by Hicks between July 2006 and February 2007, accompanied by Jeroen Swinkels of the University of Amsterdam. A new base camp was established in the Gangu Forest.[14]

In some ways, the apes behave more like gorillas than chimpanzees. For example, they build ground nests as gorillas do, using interwoven branches and/or saplings, bent down into a central bowl. However, they frequently nest in the trees as well. Often ground nests will be found beneath or in proximity to tree nests. Their diet is decidedly chimpanzee-like, consisting mainly of fruits (fruiting trees such as strangler figs are visited often).[11]

The Bili apes pant-hoot and tree-drum like other chimpanzees, and do not howl at the moon.[13]

Behavior toward humans has baffled and intrigued scientists. There is little to no aggression, yet no fear, either. "Gorilla males will always charge when they encounter a hunter, but there were no stories like that" about the Bili apes, according to Ammann. Instead, they would come face-to-face with their human cousins, stare intently in half-recognition, then slide away quietly. Hicks's group later confirmed and somewhat expanded those observations, saying that when they encountered a large group of Bili apes in the deep forests (far from the roads and villages), they not only approached the humans, but also would actually surround them with intent curiosity.[11] Hicks clarifies the issue as follows: the apes within 20 km or so of the roads flee humans almost without exception. The adult males show the greatest fear. Further from the roads, however, the chimpanzees become progressively "naive".[11]

The Bili ape has been reported to walk upright, bipedally 6 to 6.5 feet tall, with the looks of a giant chimpanzee. Their footprints, which range from 28 to 34 centimeters, are longer than the largest common chimp and gorilla footprints, which average 26 cm and 29 cm, respectively.

According to Williams, "They have a very flat face, a wide muzzle and their brow-ridge runs straight across and overhangs. They seem to turn grey very early in life, but instead of turning grey-black like a gorilla, they turn grey all over." They develop uniform grey fur independently of age and sex, which suggests that greying takes place early in life (whereas in all known gorilla species, only males gray as they age and graying is restricted to their backs).

Bili ape skulls have the prominent brow ridge and may sometimes have a sagittal crest similar to that of a gorilla, but other morphological measurements are more like those of chimpanzees. Only one of the many skulls found at Bili had a sagittal crest, thus it cannot yet be considered typical for the population. Chimpanzee skulls are 190 to 210 millimeters long, but four out of five Bili ape skulls measured more than 220 millimeters, well beyond the end of the normal chimpanzee range.[13]

Female Bili apes have genital swellings similar to other chimpanzees.[11]

The Bili Forest lies in the Congo's far north, about 200 kilometers east of the Ebola River, where deep tropical rainforests are broken by patches of savanna. Dense jungles, civil war, and other barriers to mankind's encroachment have left the region relatively pristine. However, forests throughout the Congo have been hit hard by commercial poaching.

Beginning in July 2007 the Bili area saw an influx of gold miners, putting at risk the continued survival of chimpanzees, elephants and other megafauna in the area.[15][16][17]

The Bili apes are currently under threat from an invasion of illegal poachers into the Bili area, which began in June 2007.[15] Over a 14-month period between September 2007 and November 2008, researcher Cleve Hicks and his Congolese assistants documented 34 chimpanzee orphans and 31 carcasses for sale in the nearby Buta - Aketi - Bambesa region (seven of the orphans have been confiscated and adopted).[18][19][20] Laura Darby and Adam Singh have seen another nine chimpanzee orphans and three carcasses in Aketi, Buta and Bondo since Hicks left in November.[21] In addition Hicks observed a large quantity of okapi and leopard skins along with elephant meat and ivory. It is likely that this exploding bushmeat trade is now making its way into the Bili region with the gold miners.[9][22][23][24][25]

Eikelenboom, Esther. Een Vandaag. "[1]". Cleve Hicks is interviewed on Dutch TV about his latest expedition (2007–2008) to the Congo. Includes footage of the Bili apes. The commentary is in Dutch but the interview is in English.

Hicks, Cleve. "The Bili Apes Are in Trouble!". September 17, 2007. The Bili area has been invaded by gold miners, putting at risk the Bili apes and their pristine habitat. A report by Cleve Hicks of the Bili Apes Project, at Richard Dawkins.net (also available, with photos, at the Friends of Washoe site [2])

Hicks, Cleve. "The Bili Chimpanzees" on The Wasmoeth Wildlife Foundation website - Please see Cleve Hicks's new updates on this site, 'Trading Chimpanzees for Baubles: Aketi Field Season 2007-2008' [3], "Field Season 2004-2005: Into the World of the Bili Apes", and "Field Season 2006-2007: A New Beginning". Photos and films of the Bili apes are presented to the public for the first time.

Hicks, Cleve. "The Bili Apes Are in Trouble!". September 17, 2007. The Bili area has been invaded by gold miners, putting at risk the Bili apes and their pristine habitat. A report by Cleve Hicks of the Bili Apes Project, at Richard Dawkins.net (also available, with photos, at the Friends of Washoe site [4])

Darby, Laura. [5]Tales from the DRCongo Laura Darby and Adam Singh recount their adventures caring for five orphan Buta/Bili chimpanzees in Aketi, The DRC.

Hicks, Cleve and Darby, Laura. [6]A New Home for the Aketi 5 Chimpanzee Orphans Cleve Hicks and Laura Darby recount the evacuation of the Aketi 5 chimpanzee orphans from the northern DRC to the Lwiro Sanctuary in the east.

Hicks, T. C. et al. African Primates 2010. [9]Trade in orphans and bushmeat threatens one of the DRC's most important populations of Eastern chimpanzees

IBED Report 2011. [10]A summary of Cleve Hicks's Ph.D. project on the Bili apes, and links to a number of interviews and press releases

Hicks, C. 2011. [11]Encounters with Bili chimpanzees in the undisturbed Gangu Forest. In Among African apes: Stories and photos from the field. Editors Martha M. Robbins and Christophe Boesch. University of California Press.

University of Amsterdam. [12]Bushmeat orphans from a unique chimpanzee culture

Hicks et al. 2014. [15]. Absence of evidence is not evidence of absence: Discovery of a large, continuous population of Pan troglodytes schweinfurthii in the Central Uele region of northern DRC. Biological Conservation.

Hicks, C. [16]. Max Planck institute of Evolutionary Anthropology. New large population of chimpanzees discovered.

Hicks, Cleve. [17]Save the chimps Hear Cleve Hicks describe his latest experiences in the northern Congo working with the Buta/Aketi chimpanzees, and also the enormous bushmeat crisis which threatens their very existence; the piece follows a story about the Vietnam war.

Hicks, Cleve. [18]A Mega-Culture Club in the Congo? Interview with Cleve Hicks on the discovery of a large, continuous population in northern DRC, February 2014 (scroll down).